Frequency selective amplifier



Oct. 22, 1940. A A. LuNDsTRoM 2,219,189

FREQUENCY SELECT-IVE AMFLIFIER Y Filed 0G12. 4, 193B }To OTHER sewn/Na elm-wr: F/G.3

- RECEIVING /9 OTHER li'E'CE/VINGA CIRCUITS A TZQRNE Y CAD Patented Oct. 22, 1940 UNITED STATES 2,219,189 FREQUENCY SELECTIVE AIVIPLIFIER Alexis A. Lundstrom, East Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 4, 1938, SerialNo. 233,171 7 claims. (o1. 179-171) This invention relates to amplifying systems and particularly to amplifying systems which are selective as to frequency. More particularly, it relates to the employment of semiconductors for selective amplification, and to the use of semiconductors, such as compounds of uranium which under suitably controlled conditions exhibit both negative resistance and positive reactance characteristics.

It is known that certain compounds which are or-dinarily classed as semiconductors exhibit a `negative temperature coeicient of resistance. This property has been discussed by L Blanc and Sachse in Sachsische Gesell'schaft'des Wissenschaften: Berichte Uber Verhandlungen, vol. 82, pages 133 and 153, 1930, and is the sub1- ject of several patents of which 1,526,139 and 2,017,011 may be taken as representative. These compounds may be employed to form circuit elements by fusing or otherwise binding platinum or other conductor leads into electrical contact with a suitable quantity of the compound.

These elements are ordinarily of comparatively high resistance. For very small currents the voltage increases with the current but as the current is increased a point of inflexion is reached beyond which the voltage decreases as the current increases and thus a negative slope `is achieved. It is in this latter range that these elements function as negative resistances or amplifiers in the manner well known in the art. A usual practice is to employ a direct current potential to bring the element into the region lof negative resistance, whereupon it may be inserted in a suitable alternating current circuit so as to amplify the current in that circuit.

Although this property-of certain compounds of uranium, particularly uranium oxides, of boron and of other elements is well known, circuit elements constructed therefrom have not found extensive use as amplifiers for'the reason that they are sluggish in action and of comparatively low gain. Now I have found that these effects are in large part the result of a reactive component in these negative resistances which appears to be a function of lag temperature change with change in current owthrough the resistance, and I have further found that I can neutralize this reactive component bygincluding in the circuit therewith a xed reactance of opposite sign and that by this means I am able to construct a combined amplifier and band-pass filter which will provide a considerable amplification in a selected range of frequency at a very low cost. Such an equipment has application in various selective signaling systems such as voice frequency telegraph, multi-party lines and the like.

For a detailed description of the invention and of certain preferred embodiments thereof, reference is made to the drawing in which:

Fig. 1 illustrates a basic alternating'current system employing amplification means in y accordance with the invention;

Fig. 2 is a diagram of the electrical, equivalent of the negative resistance velement employed;

Fig. 3 shows in a `preferred arrangement the employment of the inventionv for voice frequency carrier telegraph; and l Fig. 4 showsftypical gain-frequency'characteristics which may be obtained in a circuit such as shown in Fig. 1.

Referring to Fig. 1 which illustrates a basic circuit of the lter lamplifier ofy theinvention, the element T vis of a material whose resistance is temperature-controllablej i. e.,l controlled or determined by the temperature at which the element is maintained, or to which itis adjusted fromsome previous or initial temperature. -`Such an element may be of uranium oxide suitably mounted and provided with ,leadsY and may be designated as a Thermistor'fjwhich maybe defined as a device or element whose resistance, because of temperature effects, varies with the equilibrium current through it.v The thermistor is biased through the current limiting resistancey RB by means of the battery 2. This combination is connected in series with the combined tuning and -direct current blocking condenser C` thus forming the basic group of the invention.

It has been found thaty'th behavior of the thermistor T Ainian alternating current circuit is substantially equivalent to that of the network of Fig. 2 in `which a negative resistance-R1 in series with the positive reactance L1 are shunted by the direct current resistancevRo. AThis thermistor, therefore, may be considered equivalent to a network whose impedance ZT is equal to which, in known manner, may be shown to be equal to "KRO- RSM-@LTP Provided RB is sumciently large and the thermistor properly biased, the impedance Z has a negative resistance and a positive reactance component. Selective amplification is obtainable when-this reactance is combined with a suitable Anegative. reactance byA proper proportioning of the condenser C. The gain may be defined as the ratio of the current flowing through the receiving resistance Ra of Fig. 1 to the cur- B. These are plots of gain measurements made in this manner on a circuit such as Fig. l and indicate amplifier characteristics which may be obtained. In obtaining the data of curve A-the biasing resistance RB was set at 29,900 ohms, the sending resistance Rs and the receiving resistance Ra at 4600 ohms each and the condenser C at 0.09 microfarad. The biasing current was 0.003 ampere. The values of the elements of the equivalent network for the thermistor were Rrr-:46,000 ohms, L1=8.53 henries, -R1== 9050 ohms. Curve B was obtained from the same thermistor T but the other elements ofthe circuit were changed as follows: RB=22,000` ohms, Rs=RR=l ohms, 0:.093 microfarad andbiasing current 0.0049 ampere. These curves illustrate the fact that a given thermistor may be employed to produce amplification in circuits havingV a considerable range of sending and receiving resistance and also that the effective inductance L is a function of the biasing current. In general, the frequency of resonance may be shifted .and the circuit made sharply or broadly resonant by a proper proportio-ning of the magnitudes of the components as is well known in the use of circuits containing resistance, inductance and capacity. A balanced thermistor amplifier is shown in Fig. 3 as a part of a voice frequency carrier telegraph system. A source of alternating current II is modulated with telegraph signals by the key III. The modulated currents may -be passed through 4a transformer 9 into the filter-amplifier circuit containing the thermistors T, T, the biasing potential 8, the biasing resistances 1, 'I and the blocking and tuning condensers 6, 6 and thence through the transformer 5 into branch 4 of transmission line 3. At the receiving end the modulated currents come in over line 2l and are divided among the several receiving circuits. Those for which amplifier filter T', I6, I1, I8 is resonant will be predominantly led into branch 20 and thence through transformer I9, the filter amplifier comprising condensers I8, I8, bias resistances II, I'I and thermistors TyT which are biased by potential source IB. The amplified modulated signals will be imposed upon demodulator I4 via transformer I5 and the demodulated signals will then be impressed upon receiving relay I2. It is to be noted that the thermistcrs T, T are reversible, that is, that they function as negative resistance in the circuit regardless of the direction of current flow. l

These specific and preferred embodiments are indicative of the advantages to be gained by the employment of my invention` and the methods of its use. They are not to be considered to circumscribe the invention however, since it is evident that many variations and many different arrangements may be employed without departing from the spirit of the, invention-as defined by the following claims.

What is claimed is:

l. A selective amplifying system comprising a source of variable electric current, a work circuit, a negative resistance having a reactive component, and a reactance of opposite sign to neutralize said reactive component.

2. A system forfrequency selective amplification comprising a source of variable electric current, a work circuit, and a balanced amplifier comprising balanced temperature controlled negative resistances having an effective reactive component and associated therewith reactances of opposite sign adapted to neutralize said reactive component at a desired frequency. 3. A frequency selective amplifier of the balanced type comprising an input coil having two balanced secondary windings connected in a series-aiding arrangement thus having two secondary terminals, an output coil having two primary terminals, a first secondary terminal of said input coil connected to a first primary terminal of said output coil through a temperature controlled negative resistance having a reactive component and associated with a reactance of opposite sign, the second secondary terminal of said input coil connected to the second primary terminal of said output coil. through a second temperature controlled negative resistance having a reactive component and associated with a reactance of opposite sign, a battery supply circuit leading from one battery pole to the junction point of the balanced secondary windings of said input coil thence through the two branches each comprising one-half of the input coil secondary winding, one of the said negative resistances, and a battery control resistance connected to the other pole of the battery and the said reactances of opposite sign respectively adapted to neutralize the effective reactance of its associated negative resistance at a desired frequency.

4. An electric wave system comprising a compound of uranium electrically equivalent to a positive resistance shunted by an inductance and a negative resistance in series, and a condenser adapted to tune the said inductance to a desired 5. A frequency selective amplifier comprising acompound of uranium, a battery supply circuit therefor adapted to bias said compound to a condition in which it is equivalent to a positive resistance shunted by an inductance and a negative resistance in series, and a condenser adapted to tune the said inductance to a desired frequency.

6. An electric wave circuit comprising in combination a source of carrier current, and an ampliiier comprising temperature controlled negative resistance having an effective reactance component and associated therewith reactance of opposite sign adapted to neutralize .said reactive component at the frequency of said carrier current source. y

7. An electric wave system comprising means responsive to waves of a given frequency, a nega-I tive resistance element having an effective reactive component, and associated therewith la reactance of opposite sign adapted to neutralize said reactive component at said given frequency.

' ALEXIS A. LUNDS'IROM.y 

